I started implementing the driver structure and the platformspi slave functions since they are declared in WICED-SDK\WICED\platform\include\platform_peripheral.h but not implemented anywhere.I wonder if there is a workaround or if I made a mistake somewhere, considering that the snip.spi_slave is supposed to work.

Only the SPI master is implemented for all WICED platforms. The SPI slave is only implemented for select platform at this time.

The SPI slave can be added to other platforms as well by implementing the "platform_spi_slave_init(...) on the platform file. For ISM43362_M3G_L44 it would be ...\WICED\platform\MCU\STM32F2xx\peripherals\platform_spi.c